Abstract
The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.
Highlights
Al-Si alloys are important structural materials commonly processed by means of casting methods due to their good fluidity[1]
The formation of a non-uniform grain structure is prominent in directionally solidified parts such as DC-casting billets, whose diameter can be in the range of meters
As per the classical nucleation theory, more uniform grain structures can be achieved if potent nuclei are present to favour the heterogeneous nucleation of the crystals of the primary phase[6]
Summary
Al-Si alloys using Nb-based received: 08 September 2016 accepted: 23 November 2016 Published: 23 December 2016 heterogeneous nuclei. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. A typical example of the interaction of intentionally added heterogeneous nuclei with the alloying elements of the molten material does happen when Al-Ti-B or Al-Ti-C master alloys are used to inoculate Al-Si alloys These Ti-based master alloys, which are very efficient in promoting the nucleation in wrought Al alloys, are not effective in Al-Si alloys because the Ti-based nuclei react with the silicon of the Al-Si alloy to form titanium silicides (i.e. TiSi, Ti5Si3 and TiSi2)[11,12,13,14,15]. We demonstrated that Al3Nb/NbB2 can promote the nucleation of Al crystals in Al-Si alloys processed by die casting[22,23]; the formation of uniform grain structures in directionally solidified Al-Si casting has not been achieved before. The semi-quantitative EDS chemical analysis further proved the chemistry and stoichiometry of the Nb-based compounds
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